This invention relates to a process for refining a molten metal, and more particularly to a secondary refining process, wherein harmful or unnecessary impurities contained in a molten metal in the form of non-metallic components such as sulfur (S), oxygen (O), hydrogen (H), nitrogen (N), etc., and metallic components such as lead (Pb), zinc (Zn), bismuth (Bi), tellurium (Te), etc. can be reduced to tiniest traces effectively and in an extremely short time as well.
In the metals and alloys obtained by melting, harmful or unnecessary metallic and non-metallic components, in addition to essential metallic components, are often contained owing to the choice of the materials utilized. And it is well known that impurities contained in a molten metal such as sulfur, oxygen, hydrogen, nitrogen, etc. largely affect the quality of the final products. That is why a secondary refining process is often carried out, which executes an extra purification of the molten metal taken out of a melting furnace.
As conventional secondary refining processes RH, DH, FINKL-MOHR, LF etc., are widely known. Among them RH and DH methods are superior in degassing, but not suitable for promoting desulfurization and deoxidation by means of adding flux, and not capable of maintaining heat of the molten metal. Consequently it becomes necessary to heighten the temperature of the discharged molten metal in the primary furnace, for example an electric arc furnace, which inevitably sacrifices the efficiency of the primary furnace and the cost of the refractory materials therefor. And FINKL-MOHR and LF (ladle furnace) methods are capable of executing reducing-refining with their heating apparatus equipped with electrodes, and also adaptable to the gas blowing. They are, however, inherently weak in the capacity of stirring the molten metal, because the ladle itself works as a refining furnace and a porous plug is utilized. They are consequently disadvantageous in their low refining efficiency.
Impurities contained inevitably in the molten metals, in particular, in the molten steel, for example, metallic components such as small amounts of Pb, Zn, etc., are harmful to the quality of the obtained products. Degradation of the hot-workability, crack-sensibility of steel ingots, and degradation of the thermal fatigue life are well known as demerits caused by the impurities. It is essentially required to reduce, in the course of refining of the molten metals, such impurities to a low level where they do no harm to the quality of the products. According to the general belief Pb and Zn elements must be less than 15 p.p.m., and Bi and Te elements are allowed at a level of 5 p.p.m. In austenite stainless steels containing Ni and Cr, for example, cracking in the course of hot-working can be actually prevented at the content level of 15 p.p.m. of Pb and Zn and 5 p.p.m. of Bi and Te.
According to the conventional actual manufacturing method, application of oxidizing-refining to the molten metals in a primary furnace like an electric arc furnace enables removal of the above-mentioned unnecessary elements relatively easily, because they produce oxidized substances of low boiling point. In cases of not employing oxidizing-refining, that is to say cases wherein oxidizing-refining is not needed because of obtainability of the combination of required ingredients to some extent by means of proper arrangement of materials such as in high carbon steel or high alloy steel, harmful elements must be prevented by controlling the material arrangement or removed by any other methods than the oxidizing-refining from the molten metal. In the prior art removing the harmful metallic elements from the molten metal has been extremely difficult.